2Alleles and Appearance Haploid gametesDiploid zygote
3Alleles and Appearance Remember:A gene is part of a chromosome.Each inherited characteristic is controlled by 2 copies of its gene.The 2 copies of the gene that determine a particular characteristic may be the same or different.Different forms of the same gene are called alleles.
4Alleles and Appearance Remember that each gene occupies a specific site on a chromosome.Consider the gene for height in pea plants.Allele for ‘tall’Allele for ‘dwarf’What will the appearance of the pea plant be?
5Alleles and Appearance Note Alleles are described as dominant or recessive.A dominant allele always shows up in an organisms appearance (masks the recessive allele).A recessive allele only shows up if there are two of them.
6Alleles and Appearance Alleles are symbolised using lettersDominant alleles are given capital letters, while recessive alleles are given the same letter but in lower case.For example, in pea plants the dominant allele for tallness has the symbol ‘T’ and the recessive allele for dwarfism has the symbol ‘t’.
8Alleles and Appearance Notes Collect and complete the cut-out “Symbols for Alleles”, by choosing suitable symbols (letters) for the alleles.
9Alleles and Appearance The two descriptions of an organisms characteristics that you need to know are:GenotypeandPhenotype
10Alleles and Appearance Notes The genotype of an organism is the alleles it carries for a particular characteristic. Genotypes are usually written as letter symbols, e.g. Tt.
11Alleles and Appearance Notes The phenotype of an organism is the outward appearance of one of it’s characteristics and is usually described in words, e.g. tall.Phenotype is dependent upon genotype.
12Alleles and Appearance Notes More vocabulary!A homozygous organism has two identical alleles of a gene, e.g. TTA heterozygous organism has two different alleles of a gene, e.g. Tt
13Alleles and Appearance Notes Collect and complete the cut-out “Genotypes and Phenotypes in Mice”thenCollect and complete the cut-out “Predicting Phenotypes”. In fruit flies, the grey body allele (G) is dominant to the black body allele (g).
14Alleles and Appearance Think:Can different genotypes give the same phenotype?Can the same genotype give different phenotypes?
16The Monohybrid Cross Notes A monohybrid cross is a breeding experiment which follows the inheritance of just one characteristic, for example coat colour in mice.
17The Monohybrid Cross Notes Organisms which are true breeding pass on the same characteristics to their offspring over many generations.This means that the offspring of true breeding black mice are always black and the offspring of true breeding white mice are always white.
18The Monohybrid Cross Think! Will the genotype of a true breeding organism be homozygous or heterozygous?
19The Monohybrid Cross Let’s look at the example of mice. In mice, black coat (B) is dominant to white coat (b).
20The Monohybrid Cross P (parents) X F1 (first filial generation) F2 (second filial generation)Filial =son / daughter
21The Monohybrid Cross P X F1 X F2 BB BB Gametes all ‘B’ BB BB
22The Monohybrid CrossWhat would the genotype of true breeding white mice be?What would be the genotype and phenotype of the F1 and F2 generation?
23The Monohybrid Cross bb Parents x b All offspring (F1) bb bb x b Gametes allbAll offspring (F1)bbbbxGametes allbSecond generation(F2)bb
25The Monohybrid CrossAnother type of cross is when each parent is a true breeding individual, but each has a different phenotype.What would happen if you crossed a true breeding black mouse with a true breeding white mouse?
26The Monohybrid Cross P F1 F2 X X b b B B B b B b bb BB Bb Bb BB bb Bb
27The Monohybrid Cross B b BB Bb bb This can be shown in a punnet square:GametesBbBBBbbbGametes
28The Monohybrid Cross Notes Practice crosses:Pea plants can be tall (T) or dwarf (t).If a true breeding tall plant was crossed with a true breeding tall plant what would the phenotype and genotyope of the F1 and F2 generations be?
29The Monohybrid Cross Notes Pea plants can be tall (T) or dwarf (t).If a true breeding tall plant was crossed with a true breeding dwarf plant what would the phenotype and genotyope of the F1 and F2 generations be?
30The Monohybrid Cross Notes R = red flowers r = yellow flowersA plant homozygous for red flowers is crossed with a plant that has yellow flowers.Work through the cross to the F2 generation.What is the phenotype and genotype of the F1?What is the phenotype and genotype ratios of the F2?
31The Monohybrid Cross Notes R = red flowers r = yellow flowersA plant heterozygous for red flowers is crossed with a plant that has yellow flowers.Work through the cross to the F1 generation.What is the phenotype and genotype ratios of the F1?
32The Monohybrid Cross Notes There is often a difference between the observed and predicted numbers of different types of offspring.This is because fertilisation is a random process involving the element of chance.
33The Monohybrid Cross How can we tell if an organism is true breeding? If a mouse has a black coat, it’s genotype could beBB – true breeding, orBb – not true breeding.
34The Monohybrid Cross Notes A test cross is used to identify the genotype of an individual showing a dominant characteristic by crossing it with a homozygous recessive individual.
39Co-dominance NotesCo-dominance is a pattern of inheritance where both alleles are expressed equally.Offspring from a cross between two true-breeding parents have a phenotype in between each parent.
40Co-dominance – Think! In horses B = black coatW = white coatWhat would be the phenotype of a horse with the following genotypes:BB?WW?BW?
41Co-dominance and Blood Group Blood group is determined by three alleles:A, B and OA and B are co-dominant to one anotherBoth are completely dominant to OThere are four blood group phenotypes:A, B, AB and O.Work out on the board
42Co-dominance NotesA man with blood group A whose father was blood group O marries a woman with blood group AB.Draw a punnet square to show all the possible genotypes that their children could have.Which phenotype could not occur among their offspring?
44Polygenic Inheritance Variations between individuals fall into two categories:discontinuous or continuousDiscontinuous characteristics fall into distinct categories – examples?Continuous characteristics show a range of differences from one extreme to the other – examples?
45Controlled by alleles of a single gene Single-gene inheritance Discontinuous:Controlled by alleles of a single geneSingle-gene inheritanceBlood GroupsO B A ABTongue rollingRollerNon-roller
46Controlled by alleles of two or more genes Polygenic inheritance Continuous:Controlled by alleles of two or more genesPolygenic inheritanceSkin ColourHeight
47presented as a bar graph. Ranges of data can be Discontinuous dataContinuous dataPea seed shoot lengths (mm)No. of seeds1-56-1011-1516-2021-25No. of pupilsTongue rolling abilityRollerNon-rollerDistinct data can bepresented as a bar graph.Ranges of data can bepresented as a histogram orline graph.
48Polygenic Inheritance Notes A characteristic that is controlled by more than one gene and is expressed as a range of phenotypes is said to show polygenic inheritance.Include an example!
49Genetics and Inheritance Organisms in genetic experiments
50Organisms in genetic experiments Qualities of an organism which make it suitable for genetic experiments are:Short life cycle, grow quicklyEasy to breed, produce many offspringEasy to observe contrasting characteristics
51DrosophilaDrosophila (fruit flies) are suitable for carrying out genetic experiments as theyare very smallare easy to breedproduce large numbers of offspringhave a short life cyclehave many different phenotypes, e.g. body colourare easy to distinguish, e.g. males have rounded abdomens, females have pointed.Complete the RISE cutout
52Using Drosophila Notes The steps involved in setting up a cross to study the colour of the abdomen in Drosophila fliesStepsNotes1. Parents are chosen, one with a grey body and the other with an ebony body.Parents must be true breeding to ensure dominant alleles are not hiding recessive alleles.
53Using Drosophila Notes StepsNotes2. Breed the parents.This produces the F1 generation.
54Using Drosophila Notes StepsNotes3. The F1 phenotype is observed.This identifies the dominant allele.
55Using Drosophila Notes StepsNotes4. The F1 generation is self-crossed.This produces the F2 generation.
56Using Drosophila Notes StepsNotes5. The F2 generation is examined and the number of each phenotype counted.This allows the phenotype ratio to be calculated.
57Using Drosophila Notes StepsNotes6. The experiment is repeated.This improves the reliability of the results.